Wireless communication systems, such as cellular, trunked, or conventional radio communication systems, are known to comprise mobile radio communication units, such as in-car or hand-held radios or radiotelephones, and a fixed communication system infrastructure that includes a plurality of geographically-diverse base sites. Each base site typically provides communication service to the mobile units located in its respective service coverage area.
In the interest of public safety, the Federal Communication Commission has mandated that, by the year 2002, all existing cellular communication systems provide a means for determining the location of a mobile unit in the respective system to within 125 meters of the actual location of the unit for purposes of allowing emergency personal to quickly determine a person's location in the event that a person places an emergency 911 (E911) call in the system. Existing methods for determining location of a given mobile unit are known. One such approach requiring use of global positioning satellite (GPS) receivers in mobile communication units is described in U.S. Pat. No. 5,416,712 issued to Geier et al. However, such GPS receivers add significant cost to mobile/portable equipment and prohibitively add to the size and complexity of such equipment, particularly portable radios.
Another known approach for determining locations of mobile communication units is entirely infrastructure-based, thereby eliminating the need for GPS receivers in the mobile units. That is, the communication system infrastructure alone determines the mobile unit's location (typically using triangulation techniques) based on transmissions received from the mobile unit. With this approach, three base sites receive a mobile communication unit's transmission and estimate corresponding times of arrival for the transmission. Each of the three base sites then provides its respective time of arrival to a centralized location processor that determines the mobile unit's location using one of a plurality of known triangulation algorithms. The three base sites that receive the communication unit's transmission are typically the serving base site (i.e., the base site currently providing communication service to the mobile unit) and two non-serving base sites (typically two base sites that provide communication service to coverage areas adjacent to the coverage area served by the serving base site).
Upon receiving the mobile unit's transmission, which typically includes a fixed waveform, such as a synchronization waveform, that is unique to a particular mobile unit in the particular region of the system in which the mobile unit is currently operating, each of the three base sites independently computes a time of arrival estimate for the transmitted signal. The time of arrival is determined by performing a correlation between the transmitted signal and the fixed waveform. Such an approach works well in a digital communication system, such as the Global System for Mobile Communications (GSM), in which the synchronization waveform is unique to the mobile communication unit operating in a particular region of the system and is sufficiently long in duration to permit an accurate time of arrival of the transmitted signal to be determined.
However, in a digital communication system, such as the "iDEN" communication system that is commercially available from Motorola, Inc., that uses a time acquisition waveform that is not unique to a mobile communication unit operating in a particular region of the system, the above infrastructure-only-based location determining approach does not work well enough to meet the location accuracy requirements for wireless E911 systems imposed by the FCC because interference produced by other mobile communication units transmitting on the same frequency with the same time acquisition waveform can cause the time of arrival estimates determined by the base sites to be inaccurate.
Therefore, a need exists for an apparatus and method for determining location of a mobile communication unit in a communication system that does not require the use of a GPS receiver in the mobile unit and that will meet the location accuracy requirements for wireless E911 systems imposed by the FCC in digital communication systems that use time acquisition waveforms that are not unique to mobile communication units operating in particular regions of the system. Such a method and apparatus that permit a completely infrastructure-based location determination would be an improvement over the prior art.